Every day, fishers, wholesalers, and the owners of Japan's top sushi restaurants gather to buy and sell more than $21 million of the freshest fish in the world.

In recent decades, the market has unexpectedly become one of Japan's top tourists attractions. Thousands come to try to get a glimpse of the market's world-famous tuna auction, where a single tuna once sold for $1.76 million.

But time is running out to see the market. In October, the market will move from its original location in central Tokyo to a new $5.42 billion site east with improved refrigeration, earthquake resistance, and sanitation facilities.

We took a walk through before dawn — when it's off-limits to tourists — to see what the Tsukiji Market is like in action.

The Tsukiji Fish Market is located in the Tsukiji district in central Tokyo. It is bordered by the swanky shopping district Ginza and the Sumida River.

The best time to visit is before sunrise when fishers are bringing in their catch and fishmongers are preparing fish for sale.

There are two parts to the market. The jōgai-shijō is a mix of sushi restaurants and shops that sell everything from wholesale restaurant supplies to groceries. The jōnai-shijō is where wholesalers process their fish and sell to restaurants. There's also a wholesale produce market, which is the part I visited.

You'll know you've reached the jōnai-shijō when you start seeing signs like this. While the wholesale market opens at 3:00 a.m., tourists aren't allowed in until after 10:00 a.m. But business is all but done by then, so I went about 5:00 a.m.

The market is a busy workplace. These trolleys whiz down every alleyway at full speed, carrying boxes of fish from dealer to dealer. You need to be on your toes. The fishmongers won't slow down for wandering tourists.

Tsukiji Market was first established in 1935, but the tradition of riverside fish markets, or "Uogashi," dates to 16th-century Japan. At the time, the first Tokugawa shogun, Ieyasu, gave fishers the right to fish so long as they supplied food to Edo Castle. Fishers would sell leftover fish at a market near the river.

The market has become one of the top tourist attractions in Japan, but it wasn't intended that way. It's a place for business. Restaurant and market owners come every day to pick the choicest pieces of seafood for their business.

More fish passes through Tsukiji than any other market in the world. Some estimates suggest that the market does more than $4 billion in business a year.

The market sells over 480 different types of seafood each day, as well as 270 different types of produce. Harvard anthropology professor Ted Bestor, who studies Japanese sushi culture, has called it "the nerve center of a global fishing industry."

Fish are brought to the wholesalers packed in Styrofoam boxes. The fish are then cleaned and put out for sale.

The real star of the market is the Pacific bluefin tuna, considered in Japan to be the "king of sushi." It seemed like at least half of the fishmongers' stalls were cutting, cleaning, or preparing bluefin tuna. Commercial fishing has diminished the species to dangerously low levels.

The most famous part of the market is the tuna auction, which happens daily about 3:00 a.m. Only 120 visitors can view the auction each day. Most bluefin tuna sells for between $2,000 and $20,000, depending on the size and the fat and oil content, which determines the taste of the fish.

Wholesalers check the quality of fresh tuna displayed at the Tsukiji fish market before the New Year's auction in Tokyo, January 5, 2017.REUTERS/Issei Kato

The New Year's auction is a major event, with bidders paying hundreds of thousands of dollars to win the best tuna. Hiroshi Onodera, the owner of restaurant and food-service company LEOC, won the final New Year's auction at the original Tsukiji site last week, paying $323,000 for an 890-pound tuna.

Kiyoshi Kimura, who owns a chain of sushi restaurants in Japan, won the auction for the previous six years. The record price paid for a tuna was set in 2013 when Kimura paid $1.76 million for a 489-pound tuna.

Kiyomura Co.'s employee holds the head of a 222 kg (489 lbs) bluefin tuna that sold for a record $1.76 million USD after cutting its meat at the company's sushi restaurant outside Tsukiji fish market in Tokyo, January 5, 2013.REUTERS/Toru Hanai

The tuna in the market is delivered all over the world frozen or on ice less than 24 hours after being caught. Then the fishmongers have to clean and defrost the giant fish.

Next up is cutting the massive tuna into small pieces with a bandsaw. The market is a very noisy place, full of rumbling trucks, yelling fishmongers, and the constant buzz and screech of saws cutting through fish.

By the end of the work day, there are a lot of Styrofoam boxes. A forklift carts the boxes away.

Seriously, watch out for the scooters. The fishmongers have no time or patience for tourists.

After you're done touring, head to any of the sushi joints in the main market for a kaisendon, or sushi-rice bowl. Unless you've been to 3-Michelin-starred Sukiyabashi Jiro, it will likely be the best you've ever had.